1. correlation, correlativity -- (a reciprocal relation between two or more things)
2. correlation coefficient, coefficient of correlation, correlation -- (a statistic representing how closely two variables co-vary; it can vary from -1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation); "what is the correlation between those two variables?")
3. correlation, correlational statistics -- (a statistical relation between two or more variables such that systematic changes in the value of one variable are accompanied by systematic changes in the other)

1.

the unity and correlation of the sciences, or rather of science, for in Plato's time they were not yet parted off or distinguishe. - from The Republic by Plato

2.

But let us defer the further correlation and subdivision of the subjects of opinion and of intellect, for it will be a long enquiry, many times longer than this has been. - from The Republic by Plato

3.

He then seeks to combine the universals which he has disengaged from sense, not perceiving that the correlation of them has no other basis but the common use of language. - from The Republic by Plato